There are new reports that some of America's most critical and well-protected sites may be even more at risk of a devastating attack than we realized - by terrorists, deranged gunmen or hackers.

CNN has learned of three startling new reports that expose serious security problems affecting power and water supplies, federal buildings and nuclear missiles bases - the three most important public sectors that many officials warned were vulnerable to attack in the earliest days after the September 11th attacks.

Shocking nuclear missile security failure

First, at a nuclear missile site, security forces struggled to respond to a "terrorist infiltration" in a scheduled test and ended up failing miserably, according to an internal investigation. As CNN Pentagon Correspondent Barbara Starr discovered, the report found that the Air Force team may not have been able "to prevent theft, damage, sabotage, destruction or detonation of a nuclear weapon."

Federal buildings may be exposed

Elsewhere, a new investigation into security at federal government buildings says that guards haven't received proper training, especially to handle a shooting rampage. After a handful of such incidents, including last fall's shooting at the Washington Navy Yard, there are new concerns that federal offices and their employees are more vulnerable than previously thought.

As CNN Justice Correspondent Pamela Brown reports, millions of taxpayer dollars are spent each year paying companies to supply the more than 13,000 contract security guards - and many lack even the most basic skills needed.

Public utility's control system hacked

CNN has also learned that hackers recently broke into the computer network of a public utility somewhere in the United States. While officials will not release any more information, experts say it could have been a water-treatment plant, a gas pipeline or a power station.

Security experts tells CNN's Brian Todd that America's enemies have mapped its infrastructure for cyber-attack, and that rival governments and terrorist groups could have the capability to strike.